Apparatus for the multiple stage devolatilization of mass processable polymers

Abstract

Disclosed is a multiple-stage flash devolatilization process, and apparatus for use therein, for recovering volatiles from mass processable polymers, which enables optimally low pressures to be employed without the necessity for refrigeration to condense the volatiles recovered thereby, comprising separating the volatiles from the polymeric material by flashing in a first flash devolatilization zone to remove the majority of the volatiles and then flashing in at least one additional flash devolatilization zone maintained at a lower pressure, preferably as low as practically possible, to remove residual volatiles, and then recovering the volatiles for further use by feeding the volatiles recovered from the additional flash zones back into the first flash zone, the higher pressure of which allows the combined free volatiles present therein to then be condensed in a conventional non-refrigeration cooling process.

What is claimed is: 
     
       1. A multi-stage vacuum flash devolatilization apparatus for removing volatiles from a thermoplastic polymer material comprising a mass processable polymer containing said volatiles to produce a polymeric material containing less than about 0.1% by weight of volatiles, said apparatus comprising: (a) means for heating said thermopalstic polymeric material to a temperature at least above its softening point but below its degredation temperature;   (b) a first flash devolatilization zone connected for receiving polymeric material from said heating means;   (c) means for maintaining said first flash devolatilization zone at a first sub-atmospheric pressure sufficient to vaporize and remove a major portion of said volatiles from said polymeric material but which is above the pressure at which mechanical refrigeration is needed to condense the vaporized volatiles, whereby there can be produced in said first flash devolatilization zone a partially devolatilized thermoplastic polymeric material;   (d) means for withdrawing a first stream of vaporized volatiles at said first sub-atmospheric pressure from said first flash devolatilization zone;   (e) a second flash devolatilization zone;   (f) means for feeding said partially devolatilized thermoplastic polymeric material to said second flash devolatilization zone;   (g) means for maintaining said second flash devolatilization zone at a second sub-atmoshperic pressure which is below about 5 mm Hg and said first sub-atmospheric pressure of said first flash devolatilization zone and which is sufficient to vaporize and remove the remaining amount of said volatiles from said polymeric material to produce a finally devolatilized thermoplastic polymeric material containing less than about 0.1% by weight of volatiles;   (h) means for withdrawing a second stream of vaporized volatiles at said second sub-atmospheric pressure from said second flash devolatilization zone;   (i) means for feeding the second stream of volatiles from said second flash devolatilization zone into the first stream of volatiles withdrawn from said first flash devolatilization zone, to produce a combined stream of volatiles at said first sub-atmospheric pressure;   (j) means defining a source of cooling water which supplies said cooling water at a temperature which is sufficiently low to condense the combined stream of volatiles at said first sub-atmospheric pressure;   (k) means, comprising a condenser, for condensing the combined stream of volatiles at said first sub-atmospheric pressure by contacting said combined stream with a heat transfer surface which has been cooled by said cooling water; and   (l) means for supplying cooling water from said water source means to said condenser whereby said means defining said source of water, said condenser and said cooling water supply means lacks any means for mechanical refrigeration.   
     
     
       2. An apparatus according to claim 1, wherein said means for heating said polymeric material comprises an extruder. 
     
     
       3. An apparatus according to claim 1, wherein said second flash devolatilization zone is the final devolatilization zone. 
     
     
       4. An apparatus according to claim 1, wherein each of said flash devolatilization zones contains means therein for dispersing and increasing the surface area of said polymeric material. 
     
     
       5. An apparatus according to claim 1, further comprising means for mass polymerizing said polymeric material, said mass polymerizing means being located upstream of said first flash devolatilization zone for supplying volatile-containing mass polymerized polymer thereto. 
     
     
       6. An apparatus according to claim 1, wherein said means for maintaining said first flash devolatilization zone at said first sub-atmospheric pressure comprises means for maintaining a pressure between atmospheric pressure and about 25 mm Hg. 
     
     
       7. An apparatus according to claim 6, wherein said means for maintaining said first flash devolatilization zone at said first sub-atmospheric pressure comprises means for maintaining a pressure a pressure within the range of from about 30 to 50 mm Hg. 
     
     
       8. An apparatus according to claim 1, wherein said means for feeding said partially devolatilized polymeric material to said second flash devolatilization zone comprises an extruder adapted to operate under vacuum. 
     
     
       9. An apparatus according to claim 8, wherein said feeding means comprises a mixer-heat transfer device adapted for operation under vacuum for reheating said polymeric material to a temperature at least above its softening point prior to entering said second flash zone.